Recently DSS (9) invited one of his friends for a sleepover (a first for him and us) and DH and I were concerned that DSS's mother would make a fuss/retaliate by cutting access, so apart from a cancelled weekend, we were relieved there was no major fall out. (DSS's mum has MH issues that have an impact on a lot of things).
However, it turns out the cancelled weekend was a co-incidence, as DSS hasn't told his Mum about the sleepover. DH is all for keeping it quiet as well, but I don't think that is OK on a number of levels; the two mothers are bound to meet up at some point in the school yard or whatever and she will find out and feel deceived; it's not an OK message to give him, that one parent can let you 'not mention' something to the other; that he shouldn't be encouraged to think that this is a good way to deal with his mum's symptoms; and finally because it is the same as lying.
Or am I being really fussy about it? DH thinks we should leave it up to DSS.