I'll admit that I've no experience with chargers as large as you use, but my experience with chargers in the 40-50A range is that as long as the batteries will accept the charge, they can supply essentially their full rated capacity. However, for the sake of argument, let's use your 70% figure. That 25A charger could then supply 25A * 70% * 24H in a day. That's 420AH per day. As long as Max's daily usage is less than that, he should be OK with the smaller one, especially since he'll also be getting some help from his solar panels. If, when he calculates his total daily usage, it's greater than 300AH or so (I threw in another 25% fudge factor just to be on the conservative side), he should probably go for the bigger charger.

If he sits down to his computer while his wife keys up the SSB and the refrigerator kicks on and all the lights are on, his needs at the moment could very well surpass the capability of the charger. That's not a problem though as the batteries will make up the rest. They will then be recharged by the charger later.