Actually, it may not have a lot of free memory! Please remember that Apache runs a number of children at any one time (some to handle requests and others to listen for incoming requests). When you have a memory hog application running, it will limit the other children and, if one of them get the same request, it could easily cause a memory problem.
How to handle it? The first step is to contact your host (who will likely try to upsell their services - as this may be your only way out) but they may have a way to throttle the requests on the memory hog request, too.