With all due respect to your mechanic, he knows nothing about biodiesel.

Biodiesel has better lubricity than regular diesel fuel. It will not damage your lift pump in any way.

There are thousands upon thousands of people running biodiesel in everything from brand new $50K pickup trucks to 1960's farm tractors with no problems.

Sometimes people do find an occasional rubber fuel line or seal that needs to be replaced with something synthetic but the problems like that are minor in nature.