Introducing foods, constipation, and coparenting

So my almost 8 month old started eating solids around 6 months, mostly BLW so he only really ate maybe 1-2tsp of foods. His dad was refusing visits from when he was 5 months until he was just over 7 months, when he finally compromised on a visit arrangement that was safer for our son his gf reached out asking what to feed him, I was WAY more hands off than I am normally (especially with foods, I’m BIG on whole, healthy foods with no crud additives) and just said they could do their own research on how they want to do it and try to stick to single ingredient with no additives.

They clearly did zero. Started with an entire 4oz jar of bananas. Which led to him being constipated for just over a week, I had to help the poor guy out and that poop would NOT have come out without help. I told his dad that the bananas really stopped him up and if they give them again to give 1-2tsp and combine with something easier to digest. That same night they gave him 2oz of BANANAS.

I messaged the gf because I know she’s doing most everything with my son, and just mentioned (and worded very kindly, she was very receptive) typically you start with veggies and build from there and a list of what he has had at home and mentioned again that the bananas really stopped him up to the point he was screaming and so uncomfortable.

Well I haven’t asked yet what they fed him last night so I’m not sure, and maybe it’s from last weeks second dose of banana but he kept pooping tiny pebbles and STRUGGLING so I helped him again and he had a ton of super hard poop (like would be hard for an adult to pass, definitely not the peanut butter texture baby poop should be at this point, his was dry and hard) and it smelled like straight banana. so what should I say? Should I mention to just avoid bananas for now and start with a clean slate with ONLY easy to digest foods and 1-2tbs or 1-2tsp?

If so, what foods have mamas experienced helped with babes struggling with constipation? My first never had this issue as I was the only one feeding him and was able to ease him into foods and avoid this.. it also I know they are different babies and this could just be something my second is having a hard time with naturally not just because of his dads ignorance lol.