Here's a few thoughts for you:

3D Valley has a toon spider - mind you, it looks more like a woodlouse to me, and is in MAX format so won't be useful if you don't have access to 3DS Max. (This result brought to you by Mystic Nights' search page)

If it's a paying gig, It's Ming could produce something for you. They have a posable ant in their freetoon section if your friend wants to change the story a bit. :)

Maybe you could make something with Emotiguy (or Emotigal if you have her, since it's supposed to be a female spider). Parent four together, make parts invisible, morph the body etc. May be worth a try.