I also sympathise with your wife's situation. My answer will be based on my personal experience with my two girls, but also on discussions in help groups and research on the matter.
My first daughter would also not meet her wight growth milestones. She was always feeding and still not getting much weight, but met her other milestones a lot sooner.
I have been told growth does not come linear with babies, the either gain weight, or height, or milestones.
Do you or your wife know about growth spurts? The occur with babies regularily, at 10 days, 6 weeks, 3 months, 6 months, 8-10 months, 12 months, 18 months.... and do go on, even when they're not babies anymore.
Those are periods of frequent feedings, especially when breastfed, and the mother usually feels she does not have enough milk. That is because lactation is based on a supply and demand system. That means, if the baby needs more milk, he will want to feed more often, and the supply will adapt and the mother will have more milk. Supplementing with formula destabilizes this process, and the formula meal the baby gets will not be demanded from the mother and thus not be produced. Then, lactation decreases.
This is the theory. In my experience, lazy feeders like my first daughter do not manage to increase demand, and I would nurse her for hours at the 3mo growth spurt. That one was our greatest breastfeeding milestone together, and it lasted for a whole month. My second daughter was a bit better at it and I didn't feel this growth spurt as that bad. Also, from the 2-3 help groups I've been on, I noticed a trend at 3 months. Almost all mothers feel like they don't have enough milk, and all at 3 months. So it's always this growth spurt that's the hardest to overcome.
I agree with @anongoodnurse on the point that you should supplement IF the baby is hungry. And I agree that nipple confusion never happened to us, but my girls always preferred the breast, I couldn't even get them to use a pacifier... and i tried.
BUT I haven't read from your question that the baby is going hungry, only that he is not meeting weight milestones. Which could be for various reasons, and not because of insufficient milk supply.
You and only you see your baby and know if he is healthy and happy. If he is, and only points on a chart are your concern, then I really advise to at least wait out the 3 months growth spurt. Afterwards breastfeeding does get easier!
Here is a little guide to checking if a baby is getting enough breastmilk. It's on the site of La Leche League organisation. http://www.llli.org/faq/enough.html
This is from the breastfeeding clinic of Dr. Jack Newman's
I stand by my statement that the baby gaining weight and the baby being hungry are two different situations. And only the parents can asses that. Is he hungry? Ok, try supplementing if raising the milk supply fails. But why not try that first?
I like Stephie's comment, it would have been a good answer... Growth is not a point in a chart but a progression, and that is important to monitor.