The specific details aside, Lisbon is a bit remote from the rest of Europe. Even from Spain. People with cruise departures from Lisbon can find it tedious to get there from Madrid. There aren't enough flights for peak demand.
It sounds like maybe you prefer cruising, because many of the details are taken care of for you. I mention that, because if you do take the Eurostar train to Paris, you'll have to deal with arriving at least an hour early for pre-departure Customs and Immigration screening. Remember that apocryphal headline, "Fog in Channel, Continent Cut Off".
It is unreasonable for me to make suggestions for YOUR vacation, but Madrid is such a rich destination that you might consider a week in Madrid, followed by seeing your friend in Lisbon. There are plenty of nearby cities (like Segovia, Toledo, Avila ...) if you feel constrained by being in one city that's not ... London. We have used bus daytrips from Madrid to avoid renting a car, but seeing (yes, on their jack-boot imposed schedule .... but for instance, we had the option to skip the extra-cost lunch in favor of 1.5 hours extra free time) multiple sights each day, say 8AM to 7PM on the longest days. I guess I'm trying to make independent travel sound like a cruise, sorry!