I believe you've been given great advice already, the idea behind http://www.supportmymissionary.com was drawn from that very question. What can I do to "support my missionary" It's obvious you care about your friend, therefore the best thing you can do is help him (or her) be a better missionary. Help him serve with all his "heart, might, mind and strength" as the scriptured describe. Let the spirit tell you and him what that means. You will know when you are writing something that doesn't lead you both closer to our Savior.