Is she using the default messaging app that came with her S8 or a different messaging app she installed? Also, when she cannot send replies to those group text messages, is this only happening when she's using just WiFi connectivity (mobile data disabled)?
There's a long-standing issue where a) group texts are sent/received as MMS (some services can send a group text as individual text messages but the common default is as a single MMS message) and b) MMS messages often interact only over mobile data connectivity, a compatibility/transfer issue especially when it involves different carriers or platforms.
If this is happening only when using WiFi only, have her try enabling mobile data and see if that works out better. Or if she just doesn't want to enable mobile data, depending on which messaging app she's using look through that app's settings menu and look for any option relative to MMS and WiFi so it will try to send/receive MMS messages over either WiFi or mobile data (cellular).

 

No need to disable WiFi, that's not part of the issue. But since she does keep mobile data enabled than the problem isn't about anything I wrote about previously, and the whatever is going on still needs to be addressed.
What kind of error message or indication is showing that replies to group texts are failing?

 

Is a group of iPhone users, using iMessage?

 

Has this problem just started happening recently or is it a long-standing issue? If the former, can she recall anything like an app update that was applied around the time this sporadic group text problem started to occur? She can check the last updated date by going into the Settings >> Apps menu, find and open the messaging app entry, scroll down to 'App details'. If the app was updated around the time she started having this problem it could be tied to some issue the updated introduced. Try doing a 'Force stop' and then 'Clear cache' on the app, and see if that makes any improvement.
She should check the settings menu of the app too, look for anything relative to MMS or group texting and adjust any options there accordingly. Another thing to check is log into her account with her carrier and see if there are any settings or options there that related to MMS or group texting.

Otherwise, on the chance the messaging app is part of the problem, it might work out better to just migrate to a third-party messaging app from the Play Store. On the surface, they tend to have a more extensive feature set and more options to customize the user interface than a pre-installed, default app; underneath some of them focus on security and/or compatibility issues that otherwise get ignored.


As for the Apple messaging thing, there's a proprietary standards issue that prevents any kind of seamless interactions between Android and iPhone users. iMessage is Apple's default, proprietary, closed-source protocol. It's very, very feature rich and for Apple users it's a solid, well supported, and relatively secure environment to interact with each other. But Apple also refuses to allow any other platform to include any iMessage support so that's a major stumbling block for Android users. An Android app developer can't just add iMessage support, its licensing gives it legal protections from being used.
Android messaging is based on the SMS/MMS protocols. A plus being that while proprietary services, like iMessage or WhatsApp, default to their own respective, proprietary protocols, almost all of them include at least limited support for SMS/MMS. But a minus is both SMS and MMS are very dated and don't support modern standards very well. So while SMS/MMS are the lingua franca for text messaging, they're also struggling to keep up with current technology. Some tout Google's RCS protocol as being the next step for all text messaging but since Apple has already stated it won't include support for RCS in its iMessage app, that pretty much just adds it to the list of several competing protocols. A possible scenario is the Android platform will eventually default to RCS vs. Apple just staying with iMessage, with that tenuous SMS/MMS link remaining as how we continue to interact with each other (Stress 'possible' though. Even though Google is pushing it really hard, RCS is only gradually getting supported by various carriers and app developers so it becoming the definitive, universal standard is still in question).

 

I am having the same issue on my Galaxy Note 9. I can't respond to group texts, but if I enter everyone's name as if I'm starting a new text it does send the message and it shows up in the thread with other messages from that group...so I guess you would say I actually can respond, but not without reentering everybody's name... I can't just type in a message and hit send. My message just turns grey and shows an explanation mark next to it.