Any welding at all on an aluminium frame will require you to have the whole frame heat-treated to avoid weakening it. Friction welding might be marginally better than TIG, but I'd imagine it will still weaken the frame to an extent, and a downtube failure at speed is probably rather unpleasant.

That aside, correct me if I'm wrong, but doesn't friction welding involve pressing the part very hard against whatever it's being welded to and spinning it very fast? Unless you had something substantial inside the frame tube, that would probably cause the tube wall to collapse, and it would be almost impossible to get something of the correct size inside the downtube of most frames.

A better idea might be clamp-on downtube shifters, although you may have trouble finding a clamp that fits the oversize tubing which most aluminium bikes are made of, let alone the non-round stuff that some of them use these days.