I am sure it's very similar to Nigeria. Sims/topups were all over the place. There should be no restrictions at all. Bought mine (MTN Nigeria)from a street vendor and activated it right there all in less than 4 mins. Sim carrd was about $2 with $1 credit. As soon as she gets to Ghana and gets out of the airport she should be able to get a sim right away. I believe their name will be changing to MTN Ghana.
Website:
http://www.areeba.com.gh/
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile