I have a friend wants to rip down their garage and build a new one in their garden which will be at least 2 feet wider than the original

Unless your zoning or HOA police won't allow, you can build it as big as you want. Now, is there an engineering question hidden here? Footing, wiring, plumbing?

You will have to get a building permit for it, it will have to meet codes, it can not be too close to property lines according to the rules for your area. Also while many old sheds either just sit on the ground or on concrete blocks, in many areas if you tear those down , all new ones will have to meet the current codes.