Are we sure that the problem isn't with your former iPhone-owning friend? When someone switching platforms has a problem with messages it's usually Apple that is behind it, e.g. the infamous feature where if they don't sign out of iMessage when leaving Apple they will never receive a message from an iPhone owner.
Setting your phone to only send SMS/MMS should not be a problem because all phones can receive those. It's other types of message (RCS, Telegram, Signal, WhatsApp, Facebook, iMessage and all of the rest) where some can receive and some can't. So if your former iPhone owner cannot receive SMS I am sure that the problem is with her phone. Maybe she should check that she did sign out of iMessage (you can sign out from the web these days), though I'd not expect that to be the issue when you are using Android.
Anyway, "Messages" is a very generic name, so it isn't necessarily the same app on all devices. The Samsung app does support RCS (in fact the Samsung app and the Google app of the same name are the only 2 that I know do support it), so I guess the unspecified change you made was related to that? I'm afraid I've no experience of RCS myself, so can't help with anything related to that (it's not cross-platform, even on Android only some apps and some networks support it, so it's of little use to me). But as I say it's odd that she has problems with the universal, cross-platform protocols, and if these started when she changed platform my suspicion is that there's something related to that change behind it rather than a problem with your phone.