Just switched to formula, and now baby has "lumps in poo", why?

Formula is MUCH harder for a baby to digest than breast milk. That WILL cause a baby to become constipated.

What your wife needs to do is feed the baby as often as possible and pump after every feeding. A woman's body makes milk according to the demand. The more she supplements the less milk she will make. The more she feeds, the more milk she will make. Tell her not to give up!!

Doesn't sound like any need for concern. Mine was exclusively breastfed for the first 4.5 months - and the bowel movements were softer and "yellower." As soon as we introduced baby food, the "poo" definitely became darker and more formed (and stinkier!) I personally think it was especially due to the iron enrichment in the foods - because if I just breastfed her the entire day with no baby food, things went back to the softer/yellower/less stinky kind of bowel movement.

Don't worry too much. All the doctors and books I've consulted have mentioned that formula causes baby poo to be more solid than breastmilk, which causes baby poo to be REALLY runny sometimes. As long as the poo continues to come out, you don't have anything to worry about. :)

the consistency of poop from bottle-fed babies is greener and thicker than that of breast-fed babies. So it's normal for the consistency to change when you supplement.

as long as its coming out and shes not in pain I see no cause for concern.