This is a complicated matter its much like the topic of banning backstabing. I have played many a game and seen this happen. i have had allies message me to take over their lands when they were or were not at war. one time after we defeated an enemy my ally had not a lot of units left. he messaged me to take his lands because he felt that he was going to lose and would be a hindrance to our alliance pack. In his perspective it was a mercy kill.
In another round a ally attacked a nation and his armies got destroyed. there he knew that he was finished and he didn't want to let his coalition members to also die. he requested that we invade him to prevent his enemies from taking his land. he also sent resources and units. the thing is this is seen in irl in past and in current life. ww2 and ancient period wars are big examples of this action. its an act of preservation and to prevent your enemies from gaining to much power and to protect your people. however his enemy called us backstabers so eh. but the mentality of it is for-filling a death wish and prevent your enemies getting stronger.
now another situation you might seen is giving land and resources after allies taken over a enemy nation which there in lies a problem about your donation cap idea. its so hard to set what is and what isn't acceptable in a complicated matter where context is needed. 1 it could like backstabing, 2 mercy kill, 3 some kind of deal, 4 something else. the best option in your case you could report it for suspicious activities and see how things go from there. but as boris stated "If one player in a later stage of the game decides to surrender this isn't a violation of the rules."