tying into to plumbing

[Scott R.]

couple questions regarding plumbing on a remodel im doing, looking for a few quick answers...
heres a little paint picture for you guys :)

Based on that picture, black being bathtub,
blue being bathtub drain/overflow drain
green is galvanized drain pipe its tied into,

Id like to add a drain pipe for a kitchen sink to this drain,
have a handyman i found on craigslist doing some of the remodel for me, he suggest cutting into the down pipe on the tub, and tie into the drain at this spot, tieing in somewhere around where the red line is, I dont see this being a very good idea, and no way can i believe this would pass code. Problem i see is if it was to overflow, it'd overflow into the bathtub, I'd rather connect where the bathtub drain meets the main drain, something like the yellow line. this would require cutting into the galvanized piping instead of just the PVC for the tub. His response was either way if it were back up itd flood the tub, which i guess is true.

Any suggestions ?

FYI this will be my primary residence for the next few years.. dont want to have to go back and replace stuff, layout of the kitchen is being changed and this is obviously the easiest way to reroute the drain, and cheapest, I just dont think its a good idea!.