DH and ex divorced when he discovered she was having an affair with her student (she is a high school teacher). She started teaching him at age 14 but she swears he was of age when they started the affair. This has been kept secret in order to protect his two teenage children and to maintain the "mother" figure. She is still with this ex student (who is younger than my son, to give you an idea of the age difference), but lies to the kids about it (for example they found him in her house but she lied and said he was a student doing extra lessons and once told the younger teenager she was imagining things when she said she saw a guy through the window.

They have been seen around together attending events, the student is growing a long beard and seems like he is trying to look older. BM does school trips with the students and he is now her assistant, and this year will take both teenagers with her on the school trip abroad, with this guy as her assistant. I suspect it's to introduce him and sooner or later she will eventually out him as her partner but probably lie about when and how they met and his age so there is less of a gap (she is twice his age and he still lives at home with his parents)

We know that the affair happened, there is proof, and she "entertained" him in the family home when DH was at work and the kids were left with family. She seems to be a pathological liar, has a facade of being very devout religious person and respectful member of the teaching profession in a religious private school. I can't stand the deceit and secrets, and I don't think I can sit quietly while she continues this.

Kids were told that the divorce happened because they didn't love each other anymore, but this doesn't make sense because things are not amicable at all. She also tries to hint to the kids that the breakup happened because of DH not her.

How do I deal with this as stepparent, how should DH deal with this when (as we suspect) she has the kids staying at this trip with her secret BF? I don't agree with keeping secrets or lies and the teenagers have mental health issues, supposedly because of the divorce but I think it's because of the gaslighting and lying.