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).