Hi Unexplained Ways
The easiest way to understand how much water you can change in a day, really comes down to how far apart your water parameters (KH,GH,PH and TEMPERATURE) are between your new water your adding and the water left in your aquarium after draining. Yes it is bad to have poor water quality(Ammonia, Nitrite and high Nitrates). However it is also bad to have major swings in water parameters in very short periods. So as a rule of thumb people say not to change more than 50% in a day to help dilute nasties. Yet not cause major swings in parameters which lead to possible fatal shocks to aquarium inhabitants and also its beneficial bacteria.
If the water you are adding has identical water parameters and of course minus the nasties you are trying to remove through dilution. Then the water could be changed as many times as you like. A good example is that of number of aquariums, all sharing their water through one centralised sump tank. This water can be 100% exchanged many times over in a day. With no problems, due to all the aquariums sharing simular water conditions.
One other issue with large water changes where aquariums have extremely high nitrate levels. Sometimes this can cause shock in fish if it is cured too fast and is better done in a series of say 50% water changes a day, until it is lowered to acceptable levels. But I am assuming this aquarium is new and cycling. High nitrates is probably not going to be too much an issue for your friend at the moment.
Also if after you are doing water changes you are still having ammonia readings. There are a few products you can get to help detoxify ammonia, kick start bacteria and protect fish through this time. If you have a spare sponge filter or two in another already cycled tank. Put it in this tank and it will help speed up its cycle.