What is the Trojan name ? What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
What actions have you taken to try and resolve the problem ?
avast gives options, move to chest, repair, delete, etc. what action did she take/choose.
If she took a positive action (and best) to move it to the chest, I assume it keeps coming back ?
Is she getting any errors about not being able to more delete, etc. ?
She could enable a boot time scan. Right click the avast icon, select Start avast! Antivirus, Menu, 'Schedule boot-time scan...'