With Sora shifters he's limited to an 8 speed cassette, so an Ultegra 9 speed or 10 speed cassette won't work.
If the RD was Sora I'd say get rid of it (the RD is the only bad component in the Sora group), but since it's Tiagra I don't really see the point in upgrading it.
Tell him to ride the bike and replace things as they wear out instead.
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