My friend’s husband is touching my legs under the table

How to react

The husband (M 42) of my friend (F, 40) is touching my legs under the table. First I thought it was a mistake and I pulled my leg away.

The Second time I decided to leave my leg where it was and he kept his next to mine. He didn’t seem to notice the physical contact.

The third time I decided to move a bit and he did the same, now I understand that it was intentional.

My girlfriend never told me anything about their private life. I’m sure she is not aware of his actions. What should I do? Should I talk to the guy or should I inform my friend?