This explains some of the possible causes:
https://productforums.google.com/forum/#!msg/gmail/f4GTRXIVip0/2IzKs4zvAQAJ
One possibility is that the IP address she's trying to send from is blacklisted as a possible source of spam. That could have come about through her ISP allocating her an IP address which has previously been used by a spammer. So it might be worth her changing her IP address. To do so she simply needs to remove the power to her router (by unplugging it, not by simply using an on/off switch if it's got one), wait for a minute or two and then power the router back up again. (There will then be another minute or so before she can access the internet, while she waits for the lights on the router to stop flashing).
It's probably best if she checks her current IP address first, simply by going here:
https://whatismyipaddress.com/
Then she can return there after she's rebooted the router to make sure that she's been allocated a new address. Then she can see if her emails are accepted.
If all else fails, try using a different email service. She can get a new email account in a matter of moments here:
https://www.gmx.co.uk/
(Click 'Sign Up')