Is it incorrect to say "If you could"?

Is it incorrect to say "If you could"?

I tend to use the following construction often in my e-mails:
It would be great if you could...
However my German colleague repeatedly reminds me that according to the
rules he was taught, using "if" together with "could" in the way I do is
strictly speaking incorrect. I am however certain that this construction
is quite common (see here) and can often convey willingness rather than
ability.
Is the above construction strictly speaking incorrect, and is there a
better replacement for it?