Bertha's friends recently started drinking. she goes to a lot of parties with them where she is always pressured by the others to have a drink. which is the best way for bertha to resist this pressure?

she should stated firmly to her friends that she doesn't drink, if they keep persist the next time, she should start hanging out with people who don't drink

If her Friends cannot respect her decision and boundaries, it's better for her to move on and find another friends who can

The best way for Bertha not to be lead into peer pressure because of which she would really start drinking a lot would be to explain her friends that she doesn't drink. She can even perhaps tell them tshe has an allergy if she feels uncomfortable not drinking just when telling them she doesn't like to. 

Hair follicles, blood, etc.