My marriage ended while I was pregnant, my son’s now three; I have sole custody. His father has struggled with mental illness and takes medication. He smokes marijuana daily (he swears he doesn't while he has my son). He’s in therapy, and was in group therapy where he met his current girlfriend. I suspect she also struggles with mental health issues.
He’s supported financially by his parents. Previously, my son spent most of his father's access time with his paternal grandparents whom I know are responsible.
However, his father has recently been having him sleep over at his house, with his new girlfriend there. He’s stated that he isn't confident in the relationship, and has told me things about her that suggest she’s not reliable.
I've asked him to stop telling me things about her, but he continues. Now I'm struggling with knowing that my child is bonding with someone I don't know or trust, and spending more time with his father who I don't think is always capable. Perhaps I’m overly protective and too emotional about the issue. I can't stop worrying about it, but if I raise the topic it may be interpreted as an attack.
Should I say anything?
-Concerned

Yes, you must talk to your ex about anything that concerns you regarding your child – that’s the point of joint custody. As with many discussions with an ex, it’s wise to proceed delicately.
If you have no evidence of the girlfriend being irresponsible with your child, ask how HE sees things going. Example: question whether your son behaves normally around her, sleeps well, doesn’t act out, etc.
If you have a relationship with the grandparents, also diplomatically raise the subject with them. Since your ex isn’t deeply committed yet, these leading questions may help him assess his girlfriend better.
BUT do recognize that you cannot control his liaisons with other women, unless there are clear signs that your son’s affected negatively. If that happens, take the custody matter back to court with documented accounts.

A friend and I often double-date with our boyfriends. When I met her two years ago, their relationship was stable and happy. But she secretly cheated on him, then left him.
She dated the other man for months, then got back together with her boyfriend, without confessing anything. Soon she cheated again, and six months later again, without ever owning up.
Suddenly, I don't want to double date anymore or go for drinks. The first time seeing them together after her cheating is always awkward for me and I feel put in the middle. I was very close with her boyfriend; I can't even look him in the eye anymore. I feel horrible and that he's too good for her. It’s affected my friendship with her drastically, which isn't what I want.
-In the Middle

You’re only in the middle if you willingly stay there. It’s time to be true to yourself and speak up: You’re uncomfortable with her cheating and don’t want to hear about it anymore.
Of course your friendship’s affected - you don’t really respect this woman. You have completely different value systems. Sure you’d understand and still be a confidante if she no longer feels connected to her boyfriend and breaks up. But serial cheating that repeatedly betrays the guy isn’t something you find okay.
You may even serve as a true friend if you give her a wake-up call that her behaviour’s unattractive, unfair, selfish, and bound to blow up in her face someday.

My neighbours with whom we share a worn garden fence are an older couple with a dog; we’re a young couple with twin toddlers.
There’s a large hole in the fence, allowing their dog to poop on our lawn; our kids can’t play outside or risk getting into the mess. The neighbours say they can’t afford to repair the whole fence themselves, and insist they won’t approve our boarding it up. We’re also cash-strapped.There’s growing tension as I’m left angrily scooping.
-Disgusted

Both sides are waiting for the other to pay the shot, but meanwhile, your next-door relationship is starting to stink!
Resolve this quickly. Look into municipal by-laws and grants – there may be a local grant for property repair to help defray a lower cost which both households share. Or the by-law may allow you to board up the fence on your side, whether or not they approve.