I agree with Thesisfun. Co-authoring is a great opportunity and what you learn from collaboration (drafting, editing) will strengthen your writing skills for a solo paper later. Especially if the co-author has experience publishing. First papers from recent grads are not particularly stellar, honestly, as the authors are practising how to switch from a slog of writing like a thesis to a distilled piece of writing that needs to capture the attention to get past the desk-reject. However, this is assuming that you and the co-author will collaborate with the data/analysis/argument, not that the co-author wants to piggy-back off your entire work. If it is the latter case, then I can see your hesitation.