Hey Claw - Immunet is not really meant to be used as a stand-alone AV. It's light on resources and light on protection because it's really supposed to be used to augment existing full blown AV... as well as rather than instead of.
It really all depends on what is the most important consideration for your friend, and how he uses the internet. If he is more concerned about AV software being light on resources than optimum protection and tends to adopt a fairly cautious approach to online activities, Immunet would suit nicely.
If, on the other hand he is seeking optimum protection, and/or tends to live a little on the risky side where online activities are concerned... no, Immunet would not be suitable.
I don't believe Avira is any less or any more resource hungry than the other top AVs. The problem is that with his limited memory resources, he is going to experience difficulties running any AV which offers optimum protection.
His main choices as I see it, apart form upgrading RAM of course, would be; good protection and comparatively light on resources with Panda. Good protection and reasonably light on resources with MSE. Or accepting the reduced level of protection afforded by something like Immunet in favor of not using so much of his valuable but limited RAM.