I'm trying to think of how I would handle something like this if my husband were to go out on hikes with another woman... I think it would bother me because I would be wondering why my husband would want to go and spend all this time with another woman instead of with me - I would wonder why he doesn't ask me to come along, even if that other woman is married, because unfortunately the meaning of marriage to some people doesn't necessarily mean staying loyal.
It's great that you and your male friend are able to go out on hikes and your husband is fine with it, that must mean that he trusts your friend. Reason I say this is because my husband has always told me (and I the same to him), it's the other people we don't trust.
But, no worries about it now, he's not married yet... and who knows what the future will hold. My one suggestion would be to start trying to get her involved with you guys - might make her less apprehensive about it when he suggests going on a hike with you if she's not up for coming along.