It's wrong to lie, and the guilt is there, but the situation isn't actually bad. He doesnt want to be with people who know his "professional" side because of the demands and unrealistic expectations it would bring, feeling like he cant just be himself around them. While she is lying, she also is - in keeping up her lie - refraining from putting him up to any expectations.
So he is able to feel comfortable around her and be himself, and she genuinely enjoys being with him as himself. That's what she meant by having fun, not because of the lie but in spite of it. And so both people are enjoying their time spent together at face value. The lie complicates her feelings on the matter but isn't actually damaging their interactions all that much.
Souma is jumping the gun. You can pretend he has her best interests at heart, but it's clear she enjoys the time spent with the actor, and Souma doesnt actually have first hand understanding of their interactions, and so he doesnt have a good enough grasp of their situation to comment on their relationship. Yet he does anyway, and we all know it's NOT for her best interests but because he is jealous and acting territorial. I really dislike people like Souma who inject themselves so aggressively into other peoples' relationships, especially when they've never even met the other person.