I'm only quoting from op's own posts here
"He talked about her a lot initially" mentionitis
"He goes out of his way to help her with things."
"He gets a bit defensive about her."
"She seems to know a lot about him and the minutiae of his life" which shows he's overshared
"He used to talk about her more in the beginning but not so much now" mentionitis wearing off usually indicates things are getting serious
"He said that he feels sorry for her and wants to help her if he can (she’s having relationship problems)" classic knight/damsel in distress dynamic
"he doesn’t think she’s attracted to him." Odd thing to say rather than reassure op he's not attracted to her
"they would never let their friendship become anything more" again odd thing to say and as per pps suggests the dh and his friend have discussed the possibility
"I think he likes to be needed" he likes having his ego stroked and that's what he's getting from the friend
"he can’t just abandon her." Even though his wife has raised that this friendship is toxic to the marriage
"he’s stormed out." Hardly the most sensible response running away from the issue plus as per pps perfect excuse to get a chance to delete anything incriminating on phone and give friend/OW a heads up
"I haven’t met her husband and she rarely discusses him in front of me." For all op knows they could already be separated and when husband goes over there they're alone together
"My husband came back after about 20 minutes and went straight to bed." Still running away from the issue and not reassuring op
"He’s not prepared to give up his friendship" even though his wife - which is where his loyalty should lie - is pointing out this is a threat to the marriage. Note he didn't even offer to dial back the friendship not even on a temporary basis
"So he smiled, looked up and started listing things like she’s so good, would never hurt anyone, is one of the kindest people he’s ever met, etc." Completely focused on the friend rather than reassuring wife again!
"The look on his face. I’ve only ever seen that look before when he’s looking at our children." That would have been final straw for me to be honest
UnderMajor - frankly you seem to be ignoring the emotional affair aspects too. He IS seeing the friend alone/without wife and he is confiding in and supporting the friend to the detriment of the marriage and refusing to compromise on that at all.
I think also these days a lot of people who are innocent but accused of cheating would voluntarily show their phone records etc. He seems to me (and others) to have gone out of his way to prevent that and to try and prevent the op from even asking about the friendship UNLESS it's in such a way as allows him to speak about the friend in such a way as to demonstrate the pedestal he's put her on.
No you can't stop someone who fully intends to have an affair from doing so, but I do believe it's possible to prevent one where they're unaware or in denial that they're overstepping boundaries.