My mind

Litlegrl​(sub female){Dragon11} - This is a hard one to answer. Admittedly I have trust issues also. There are a few things your friend needs to consider. First, does he seem sincere? Listen to both what he is saying and how he is saying it. Second, has he ever "broken trust" before? If no, then this could be a one time thing. Also, I put broken trust in quotes because it is what she considers breaking trust, not what everyone else considers it. Last, is he worth trusting again? This is a hard one to answer so it needs to be thought out. She will have to be completely honest with herself and let the past hurt go. If she thinks he is worthy, she will need to put her trust in him.
I wish your friend luck and will keep in thoughts that it goes well for her

Radiance​(dom female) - I am married and have subs online, But if your partner knows your playing online that's perfect if they don't to me it is cheating still. I've told my hubby sub about the online subs I've never hidden away my life offline to anyone, You just have to be honest. If this Dom had done role play he had known the sub longer then a day, no one does role play on there first hello, unless your desperate and easy. Once a cheat always a cheat for me

SSG{ENM-TLP} - IMHO, cheating is participating in any intimate activity (emotional or physical, in person or online) without the knowledge of your partner. Personally, I think the D/s relationship dynamic is a little complicated anyway, especially if a D has more than one sub. If they had a specific agreement to be mono, then involving someone else in His play without her prior knowledge could be considered a betrayal. If it wasn't specified, I think the overall nature of BDSM gives a broad road for either partner.

youwillbemine​(dom female) - Thanks for your input ladies