There is ALWAYS ONE who comes out with this stupid, nonsensical, LUDICROUS bullshit question! 🙄
Of course the OP wouldn't care if it was a male friend, because the male friend is not likely to want to shag her husband, and he wouldn't want to shag HIM. Even if the male friend was gay, the husband very likely isn't.
Stupid fucking tired old chestnut, comes up on here every single time! 
@Saladdays01 Of course YANBU. She is after an affair and he is enabling her. This is the early days of an affair. You need to nip it in the bud now and tell him to stop the constant messaging. If he won't there isn't a lot you can do, but I would be getting my ducks in a row! I would not tolerate this shit.
As a pp said, the only woman my husband goes out for a drink with (alone) is me. Like fuck would I tolerate him going out with another woman alone, and messaging back and forth. Fuck that shit!
Tell him he can keep messing about with her as much as he wants if he wants to end up divorced. Say 'at least you can talk to her and see her as much as you like then, as you will be a free man.' Don't be that wife who is doing everything in the home and running around after a twatty husband, whilst he is pissing around with another woman, and making her his 'best friend!' Constantly messaging, and meeting up alone together.
Of course married men and women can be friends, but close friends and constant chatting privately and going out just the two of them??? Bollocks to that! Like fuck is that 'just friends.'