I would suggest that she have her own account that is associated with her new Garmin device. Technically, you can have multiple devices associated with one account. However, you can't have multiple accounts associated with one device. Hope that makes sense.
Perhaps we both misunderstood the query - looking at it again, perhaps gundogs askes whether his wife can use her Garmin Connect account (linked to her device) on his phone.
If it is that case, then technically it is possible, though not in a very comfortabe way. One possibiily is logging out of the GCM account (Menu » Settings » Sign Out), and then logging back with the other account. Another possibility is creating two different Android profile/accounts on the phone. For example by using the Android for Work feature, or a similar tool, some Android phones may offer multi-user environment also in other ways.
Another possibility is using just syncing over USB to Garmin Express on a computer for one of the accounts/devices.
