With babies I would normally work with 1 drop to 10ml if blending (0.5% dilution) in oil, but as these little ones are still very small, I would probably use 1 drop to 20mls. For the compress, again, as they are still tiny I would be more cautious and use one drop of Roman Chamomile, blended with a little full fat milk in 500ml of water and definitely do a patch test first. Unfortunately this isn't going to be a quick fix and will be a bit tedious doing it regularly.
Both my boys had colic, and it is a nightmare. Is Mum breast feeding? as what she eats may affect them. I'm not a big fan of medicating babies, but I did find that Infacol and Colief really did help. I'm sure her health visitor and other Mum's are giving her lots of advice, but do reassure her that colic does pass. Also as Candie mentioned Cranial Osteopathy is brilliant and if you can get a good recommendation I would give it a try.