Is she planning on sending through a @gmail.com or @outlook.com address?
If this is the case she can do one of two things:
- You can use Mailgun's sandbox domain service. This option is preloaded in all accounts and does not require any domain verification. However there is a send limit for 300 emails per day.
- You can use another domain you have access to and tie it to that account, this option does not allow for reputable sending and their emails will likely go to spam.
My recommendation is a third way, buy a domain to send emails from, most of the time they are around $10 a year and it will add credibility to your business.