I can sort of see why MIL may have gone with a cake vs. single serve thing. If there are three people going out for ice cream, MIL, DH, and BBF, that's three potential people having ice cream cake. Each probably had a slice. If everyone wanted cake, why buy three separate single serve pieces of cake?
I can totally see MIL and DH taking baby in for a treat on her birthday and things sort of snowballing. From a store employees perspective, I would often ask questions as someone was buying something....
Employee: Oh I see someone is having a birthday!
BBF: Yay it's my B-Day
MIL: Yup we came to get a little treat for the b-day girl
Employee: How fun, we should sing to her!
MIL/Dh: Ummm ok, fine.
Thats not to say that this situation didn't happen because of a larger issue of not respecting boundaries....But I certainly don't think there is malicious intent for the MIL to top SBF's birthday celebration for her kid, then a collusion between MIL and DH trying to get BBF to lie to her mom and hide the extra b-day celebration. I guess this is one of those situations where I wouldn't apply malice when it was possibly stupidity or just plain a snow-balled situation.