That depends on your hot water bottle, the specifications of your memory foam mattress as well as how you intend to use them together. For example, if you lie on your stomach with the hot water bottle on top of your back, then there is no problem. There is also no problem if you lie on your back with the hot water bottle on your chest (e.g. to keep your lungs warm when you have caught a cold). Basically, as long as you keep your body between the hot water bottle and your memory foam mattress, there is no problem.
But what if you intend to sleep normally on your back, but with the hot water bottle under you, i.e. between you and your mattress? Then you need to make sure the temperature of your hot water bottle does not exceed the maximum tolerable temperature of your memory foam mattress. This fact is true regardless of whether you use a mattress made of normal foam or one made of memory foam.
So what is this safe temperature? Unfortunately, there is no way to make a universally applicable statement. Different manufacturers have their own formulations of memory foam. This gives the foam different characteristics, e.g. a different response to temperatures. Some will become too soft when the temperature rises 5 degrees above normal body temperature. Others may handle a 10 degree or even 20 degree range gracefully. The only way to find out is to check the specification sheet for your mattress, or directly call up the company to enquire.
Keep in mind that this remains true even if you are using one of those Japanese-made chemical-based heat packs and not a hot water bottle. They both produce heat, and that extra heat will interact with your memory foam mattress.