People, catch your snap.
He has built a shop on a platform called "spreadshirt". This success has allowed him to open his own site, but his old spreadshirt store is out ranking his new site. He went in and emptied the spreadshirt site, but it is still ranking.
On the spreadshirt site, he is just one store of many, and he only has control of his own store, not the main site.
OP.
If you have dropped the spreadshirt site, it will eventually drop out of the index. If spreadshirt has any snap, they will automatically re-direct any orphaned links to their main site or another page in their site. If this is the case, they are gone from your inventory.
I do not know their terms of service, but if you could re-establish the store and link from your store pages to your site product pages, you could siphon the buyers off spreadshirt to your site, but I bet that violates their TOS.
You might be able to use an html redirect or a php redirect for the individual pages, but that depends on if you have access to the page source.
You may have to simply start over.
Here is another possibility. Set back up the spreadshirt shop, and use it to adertise your websites new designs, You can go with a "available only at
www.your shop.com and here at spreadshirt " for some of your designs to get your visitors to travel over to your new site. Having a bigger collection of the shirts on your own site will gradually wean your customers from the spreadshirt site. Just use spreadshirt as part of the sales funnel.