My mother-in-law's boyfriend is a gold digger

Met my husband through an international dating blog and at first, my mother in law did not approve of me. She thought I was some gold digger who was only after her son's money. For some time, she really despised me. She did not know that I managed my own businesses. When she found out about my family background and my business investments, she suddenly became nice. Her son and I got married two years ago.

A few months ago, she introduced us to her boyfriend who is a bit younger than her. I didn't think bad about him at first but now it looks like the real gold digger is here. I heard that the boyfriend keeps asking my mother-in-law to buy him and his kid expensive watches, shoes, gadgets, etc. and I also heard that he is still married to his wife and sees her from time to time. I only heard this from my mother-in-law's best friend and I know she's a reliable source. She is quite concerned as well.

How can we tell my mother-in-law that her bf is just using her for money? I mean it's quite obvious but she's in denial and we're not sure what to do.