(a) He helped his mother up to the sofa. -- Not likely unless the sofa was on a raised platform or something like that. If you mean "helped her to sit down" then you could say "helped his mother onto the sofa".
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Vincent TeoThanks. Can Isay,
He helped his mother to sit (up) on the sofa.
Yes, but only if she is lying down on the sofa and needs to be helped into a sitting position.
cwtcha) He helped his mother get up and to the sofa.