yes. you can use it and sign your own signature.
She (my boss) always uses mine (gold card to save extra fee).
The best way is to ask him to sign a note in a small piece paper
to authorize you to use it.
For example,
To whom it may concern:
This is to authorize XXXX with the following signature
____________
to use my "VISA" Card 6140 2123 1234 6789.
_____________
XXXXX