I agree with Mac-- to a point. (And realize that some of this is the innate disposition of our respective children here, because I can't say that I wouldn't do exactly the same thing if her DS were
my child instead of my own DD.)
That is, we also weigh "how much does this mean to _____{other family member}" relative to the ACTUAL risk it poses to the allergic person. If the risk is fairly low, and it means A LOT, then we've done it. We've gotten Chinese/Indian take-out and eaten it on disposable plates and basically decontaminated everything afterwards. I have even occasionally been treated to (or treated myself to) Almond Roca, which is LOADED with cashew contamination-- it definitely "stinks" to my DD, so we know...
The key to doing things like that is, IMO...
a) it can't cause the allergic person PAIN or suffering-- of any kind. I generally run such things past my DD to make sure that she's okay with it now that she's older... and when she was younger, I didn't bring stuff that seemed that appealing into the house to start with. (I'd eat it elsewhere, as others have noted.) But we are all multiply allergic, and we all
like one another's allergens, to some extent. As long as it doesn't pose a risk of anaphylaxis, we tend to view such things with a fair degree of permissiveness. I've BOUGHT my own allergens for DH and DD.
b) because of DD's threshold (VERY VERY low), I have a firewall between allergens (including may-contains) and FOOD PREPARATION FOR HER.
That is really what I see as problematic in the OP's post-- if (and she didn't say that she WAS doing this) she is eating them WHILE preparing food for the allergic person. I can think of all kinds of 'what-if' things that would very easily lead to contaminated food in that scenario. I keep my stash away from our standard food-prep areas. I would be terrified to eat ANYTHING from my kitchen if I handled our fish food in there, and I consider it compassionate to afford DD the same level of consideration there that I'd want myself in her shoes. KWIM?
My answer is, therefore, a complicated one. How important are M&M's-- to
you? Secondly, how important are they--
to your allergic child? (That is, are they an emblem of all that s/he CAN'T have/do? Or 'just candy that stinks'?) Finally,
how low is the allergic person's threshold? (If cross-contamination that seems impossibly small has resulted in a scary ER trip, then I'd rethink it, but if s/he eats in restaurants and is regularly AROUND the allergen without any ill effects, then it might be pretty low risk.)