I would ask DH to go shopping with me. Feign a bad back or something. Just tell him you need a little help. Just try to get him out with you. Let her see you going out as a couple. This will eat you up until it either blows over or it's sorted out somehow. The last thing you need to happen is that it comes between you and your DH.
What to do with immigrants? - ask Shakespeare
Reducing contact because my heart just isn't in it.
"Old fashioned" expressions that young people have never heard.


. I'd noticed that this dainty wee barmaid (I have never been dainty or wee) was doing quite a bit of flirting with him and when one of the customers 'light-heartedly' commented that 'M' seemed to have a fancy for DH, I took matters into my own hands. That evening, we visited the chippy after he'd finished working, and who should be there in the queue but 'M'....
. I sidled up to her, and gave her a very sharp kick in the ankle. No-one noticed anything except her. Now, of course, I would never do anything like that nowadays - and wouldn't ever advocate violence....but it did work 45 years ago.....no more problems. 

or both of them for drinks or a meal and try taking the wind out of her sails that way. Or flirt outrageously with her DH.
If you are worried enough to post here then it is an issue! Have you mentioned it to him in a light hearted way? Does he have a track record of excessive friendliness? I would be tempted to take him with you or give him important jobs to do for a while to break the pattern. Key point, do you trust HIM?