kapke: If that 7:57 departure time is too early for you, it's still possible for the two of you to do your trip later in the morning for €29 on a saver fare.
Sample schedule for 7/30:
Lv Boppard 8:51 (MRB regional train)
Ar Koblenz 9:08
Lv Koblenz 9:18 (RB regional train)
Ar Linz 9:52
Lv Linz 14:24 (RE regional train)
Ar Cologne - Köln Hbf - 15:22
There is currently a 10:51 departure from Boppard to Köln Messe (just across the Rhine from the main/Hbf station) that hits the same destinations and can also be bought for €29.
But to get such a ticket, you will need to do this at the DB itinerary page:
1.) Make Düsseldorf Hbf your final destination - not Cologne
2.) Schedule a stopover in Linz(Rhein) (I used 4:00)
3.) Schedule a second stopover in Köln Hbf or in Köln Messe/Deutz (I used :50)
I'm sure you'll pull up the same trains that I got if you do this. You'll also get a final connection from Cologne to Düsseldorf on a high-speed ICE train - a train that qualifies you for the saver fare, but a train you don't need to set foot on since you've made it to Cologne on the regional trains already.
Remember that you can use ANY of the regional trains between Boppard and Cologne at any hour within the ticket's period of validity - so you can leave Linz earlier or later than the schedule on your ticket indicates.
Saver fares go up in price as tickets sell - so as long as you're sure about your travel date, it's better to buy earlier than later.
Or if the €15 you'll save doesn't seem so important to bother with, just buy the full-price regional ticket at €44.60 on 7/30 when you reach the Boppard platforms.