I think the fact that he’s changed his mind about living with you, says he’s not as serious about the relationship as you.
She doesn't say he's changed his mind. They weren't planning to move in together until next spring and it's currently this summer. Next spring is months away and he's helping out a friend in the meantime. He already has a lodger and the friend is currently job hunting. This is only intended to be a temporary measure.
OP, I cant comment on this man, your relationship or his friend. No one can because we don't know him.
But a few general things.
It is fine for him to go for a walk with a friend and not tell you in advance. Its even OK for him to do that when you're not feeling very well. And it's OK for that friend to be a woman.
My son is 26. Prime 'arsehole' age for men! His best friend is female. His current flatmate is male but his two previous ones were female - friends from school and university. Both he and his flatmates have had relationships with other people during this time. He has never cheated on anyone. It's just not in his nature. Not all men are actually bad and looking to deceive their partners.
He and his best friend do a lot together. They know everything about each other and have a holiday booked together in the autumn. There is nothing else between them.
He asked me a few weeks ago if I thought his bf being female could cause problems in relationships (it hasn't so far). I was honest with him and said it could but he just needed to be honest with any woman he dated. Introduce them so they know each other as people like you would with any friend. And I would say the same to you.
If you are uncomfortable with the situation, tell him. If he intentionally (rather than circumstantially) keeps you apart, that would be more of a concern to me.
You're not in competition with this friend for his time, attention or consideration just because she is female. And she is not an automatic threat to your relationship either for this reason alone.