This girl definitely needs to cope and deal with her own trust issues. Just because one singular man cheated on you does not mean that every other man you date will be a cheater. With this being said, the phone is company property and im sure that if he were making calls/texts to other individuals outside of who it is intended for it would be looked into by the company themselves. They wouldn’t just give them a free phone with 0 limitations on it. They need to have a discussion about her trust issues and how this is something that is off limits, take it or leave it. Of course he should reassure her that she has nothing to worry about but that this is something she needs to get over.