Switched a outdoor gfci for a working one but now other components in the house don't work?
For now: cap off breaker 7's hot and neutral in the panel
What you discovered with two breakers feeding power to the two sides of a GFCI is something that basically shouldn't have ever happened to begin with; GFCIs aren't meant to have power backfed onto their LOAD terminals, even, as that can fry older units, and will cause new GFCIs to &quot;lock out&quot;.  Fixing it properly will require quite a bit of detective work (I suspect that the two circuits' hots got joined together inadvertently somewhere), but there is a way out of the jam for now.
What I'd do in this situation to get the house functioning would be to remove the hot and neutral that landed on breaker 7 at the panel from their lugs and cap them off individually, then connect the breaker 3 feed to the LINE side and the former breaker 7 feed to the LOAD side of the new GFCI.  Having the owner follow up on the situation would be highly recommended, though, as the extra load may cause some tripping of breaker 3 from simple overload of the circuit.
You'll also want to test the GFCI to make sure it's protecting what you expect it to; if the GFCI-protected loads turn out to be on breaker 3, then you'll have to reverse LINE and LOAD on the GFCI, and interchange the roles of the breaker 3 and breaker 7 homeruns in the panel as well.  Note also that this assumes breaker 3 and breaker 7 have the same amp rating; if they don't, then that throws an additional wrench into the works.