Probably just a phase for her, she might not have any real interest then you've turned your car (or bought another one) for nothing. There was one point where my wife "wanted" to drive my C10. We've been together 16 years, she's driven it once.
Want her to really enjoy Mustangs? Get her a super nice 2011-2013 V6 Premium Pony Package Mustang. Cool looks like the older ones, 300 HP, most are autos, super clean cars can be found for under 10K. If it was a failed experiment, a year later, it's still worth almost what you paid for it.
2011 Mustang 3.7L Premium Pony Package
2015 Silverado 5.3L 2LT DC Z71
1965 Mustang C Code Coupe 289/T5/3.25
1967 Chevrolet C10 350/700R/3.73
1968 Pontiac Firebird 400