Erich,

If you aren't a member of the household, it will be more difficult for the tiel to become familiar with you to the point of feeling secure. Difficult, but not impossible.

When you visit your friend and her bird, you'll need to socialize with the bird to establish some type of relationship. The socializing should include talking to the bird in a manner that will transmit a calm and secure tone.

Offer your hand for the bird to perch on but don't force it. Patience and persistence are the key when trying to change any behavioral habits. Avoid sudden moves that may startle the tiel.

The bird accepting you can take some time, but if you really want this to happen, patience, kindness, and persistence are the best methods.