score:4
UK > India > Thailand (1 hour flight crossover, in same airport) > Vietnam > Thailand (staying for 5 days) > UK
Sorry for being not very well informed about what is a crossover flight but that 1 hour stopover surely means its on the same flight booking (please cancel the ticket if not because 1 hour is just not enough to change flights on different bookings)
That means she is Transiting Thailand and for a short transit there is no visa required in Thailand
Foreigners, who travel via Thailand to other destinations from the same port of entry, are allowed to transit without a visa.
Here is some info from Timatic
TWOV (Transit Without Visa):
Visa required, except for Holders of onward tickets for a max. transit time of 12 hours through Bangkok Suvarnabhumi (BKK). Passengers must have a connecting flight booked on the same ticket.
Personal Experience
I have travelled on Thai Airways to multiple countries on the same ticket. We used our single entry Thai visa on the first leg of our journey and on our way back we had a 6 hour transit at BKK, there were no immigration steps involved that would've required a new visa.
For the sake of completeness
Bulgarian Nationals can get Visa on Arrival in Thailand. Although in this case that will not be required