From what's evident on the WeChat Developer Site, there is a WeChat API for integrating "WeChat SDK with your iOS and Android apps".
Specifically, I am looking for a way to check to see whether two accounts are friends on WeChat (I have access to the login information for one of those, the other of which varies). Is there a Web API for me to do this with? I was thinking something along the lines of REST, JSON, etc., though I can't seem to find anything of this sort.
Perhaps you are looking for the Official Account Admin Platform. I'm not sure about the whole friendship status thing, but that's the closest thing I can find to a WeChat REST API.