If you use the minus sign before the number (as in -50), you'll make them enemies -- it will take the relationship bar into the red.
I also think the capitalization is important -- often is in computer coding (I've done a bit of that in my day).
So to modify the relationship you'd first type
testingcheats true
then
Modifyrelationship Arvin Spencer Marshall Cross 50 Friendship_Main
and it should work.
Oh, and if you're not averse to mods there's a good one at Mod The Sims which disables the relationship decay; there's also a version where you can tune the percentage of decay where it does drop but not as drastically.