Well, one of my dearest friends came about in this kind of situation really.
He had been a client for a good while, but not someone I'd really come into much contact with. About 18 months ago, he gave me some help on a project I was running, and contact was initially all work based in work hours. However, the more we chatted, the more the conversation got onto personal lives - what we were upto at the weekend, music we liked, films etc etc. Over the course of a couple of months, emails were all personal with an odd reference to work. Then we exchanged numbers, and messages were all personal.
We have 2 things massively in common - same sense of humour and love of music. With music, he filled USB sticks several times over with stuff he (rightly!!) thought I'd like, and I did the same for him with films and some music. We've always liked a bit of banter and taking the piss out of each other etc.
We talk to each other about things in our lives, whether thats work stuff, relationships, family etc. Sometimes I want a male perspective on things. But above and beyond that, I value and respect his opinion and thoughts.
We've been for dinner together, to movies etc - just as mates. Nothing has ever happened, we just don't see each other in that way. For the most part of our friendship, we've been in relationships, and it really hasn't been a threat or problem.
For that reason, I wouldn't be put out by your DH's friendship really.