Is it cheating?

So i know someone who is a kind and a good hearted person. Lets call her M. she got married some 2-3 years ago within relatives. Their chemistry did not match at all which lead to frequent fights between them. Their physical relation is also not good. So the M found a friend named D (female, who is also married with kids and in 40s) and got a very good relation with her. Her husband and saas dont like her friend D as she often come to meet the M. But still she comes every month to meet the bride as M had a baby recently. Now i come to know that when M and D meet they usually lock the door and do some stuff behind the doors. M husband does not know that. He is a douchbag but still isnot a cheating even if its with a girl?

Update: Yes guys they both are girls and its not gossip, they do have physical intimacy, although to what extent is between them. And i am not gonna tell anyone bcs its their life and choices. I was just curious to know how other ppl percieve this. And for me its very shocking too that both have husbands and they both were not into this stuff before marriage. Another weird thing is M is 25 and D is around 40 so thats a weird lesbian couple