Yes, tell them. Particularly for the person who is 39 weeks and might give birth any day.
Lljjkk - the reason to do so is given here by the NHS website (I've pasted it below). The main thing is that the mother knows she was exposed so if her baby did develop CP shortly after birth, doctors might think of that before numerous other reasons a newborn baby was sick. Because it can make newborns very, very unwell.
NHS advice on chickenpox:
"If you catch chickenpox up to 28 weeks of pregnancy, there’s no evidence that it increases your risk of miscarriage. However, there’s a small risk that the baby can develop foetal varicella syndrome (FVS).
FVS can damage the baby’s skin, eyes, legs, arms, brain, bladder or bowel.
If you catch chickenpox between 28 and 36 weeks, the virus stays in the baby's body but doesn’t cause any symptoms. However, it may become active again in the first few years of the baby’s life, when it causes shingles.
If you catch chickenpox after 36 weeks, the baby may be infected and could be born with chickenpox.
Complications for the newborn baby:
The baby may develop severe chickenpox and will need treatment if you get chickenpox:
- around the time of the birth and the baby is born within seven days of your rash developing
- up to seven days after giving birth."
The chances are that the OP's friends won't be affected at all. I've been exposed several times in my current pregnancy. But it's always good to know.