As per GOV.UK:
Children aged 17 and under travelling to Portugal alone or in the company of a person who is not their parent or legal guardian, must either:
be met at the airport or point of entry by their parent or guardian, or carry a letter of authorisation to travel from their parent or guardian.
What is the best method of enabling a 17-year-old travel to Portugal with an 18-year-old friend?
As per GOV.UK this letter of authorization should:
name the adult in Portugal who will be responsible for them during their stay and
the identification and contacts details of the parent or guardian
In this case, the 18-year-old friend can be named as the responsible adult.
The potential consequences of ignoring this:
If you travel with a child without consent from their parent or guardian, you may be questioned on arrival and your entry to Portugal may be delayed until further checks are made.
Template letters can be drafted from a reputable legal template.
By having your travel permission letter notarised, you are guaranteeing the authenticity of the document. It is recommended that you have your form notarised for a minor child travelling internationally.