The baby probably slept because their tummy was full of heavy, difficult to digest food. That's not necessarily a good thing.
Agree this will probably do no lasting harm, but would feel betrayed and really angry and let down if this was me - as others have said, it's the fact she's introduced something new, whatever that might be, without asking you first and in fact in this case when she already knew the answer would be no.
I expect she was thinking you'd turn around and thank her since she'd found the solution to sleeping for you. Like your initial reluctance was just new-parent anxiety. Once she revealed that her solution had worked and didn't have any negative effects, you'd be grateful for it. She misjudged that badly.
I also think that as others have pointed out she's underestimated the seriousness with which modern parents see giving solids early, and overestimated how much the lack of sleep is bothering you. Times change - giving solids early is now a no-no and it's generally accepted that babies sleep poorly until they are bigger and that this is not something which necessarily needs fixing or sorting out. In her generation it was normal to give solids at 3-4 months and sleep was considered something to get sorted ASAP.
Unless you genuinely think it was malicious in some way it might just be worth reiterating with her that she needs to check before she does anything new, especially if she knows you're concerned about the subject. I do think it's probable this came from a place of love/concern and was simply misjudged. I suppose you'll find out by her reaction to your reaction - if she acts all defensive and like the victim then it would be difficult to trust her again. But if she is upset and feels badly that her gesture went the wrong way, then there is probably room for discussion there. (Initial defensiveness followed by later genuine apology is probably the same too.)