Is it permissible to help a friend in debt who owns a modern car and buys designer brands, while my financial circumstances are difficult and I do not feel completely comfortable with this assistance?

If your friend is in need of help, you should not feel embarrassed to assist her, for every good deed is known to Allah and rewarded by Him. If she observes extravagance from her, she should advise her. And if she finds someone more in need, then helping that person takes precedence to seek the complete reward. One should not feel embarrassed to do good and help the servants of Allah, for that brings Allah's aid to His servant.