https://www.mumsnet.com/babies/weaning This link is pretty thorough on the topic.
The general guideline is that before six months you run the risk of the baby's digestive system not being ready, but if you do wean early, 17 weeks is the absolute earliest. At this stage it needs to be thin, very smooth purees - our infant feeding consultant says like double cream.
If the hungry baby milk upset his bowels, I'd be concerned that solid food will also do that, since he is so very young.
The signs of readiness that are considered important are that he can sit unaided, he has lost the tongue-push reflex (where he pushes food back out of his mouth), and that he can grab food and put it in his mouth.
Here's some info from the NHS, though not as thorough as the first one.
https://www.nhs.uk/start4life/Pages/babies-introducing-solid-food.aspx
No judgment for her looking into this, but I would urge her to get the official advice from her health visitor/doctor (don't know where you are in the world) since I know that there are more limits on what you can feed them that early and how.