"I have a friend" questions! ....wink wink! haha
First, you have to understand one thing:
It's business - not personal.
From this perspective, you can make better decision that are more beneficial to you and the perspective company.
First, it sounds like there are a few different conflicting ingredients floating around here, that when looked at separately don't taste well - but lets stir them together and see what we have here!
To start, it seems like there is an underlying issue here with the salary and that if the pay was more - all this would probably be over looked, am I right? So, before turning it down, is there a reasonable salary you would be happy with that they might entertain? It sounds like at this point, you don't have anything to loose, so before totally closing this doors and walking away from it, try to negotiate a higher salary, if you can properly justify it. However, the second interview would be required before you can do this, you don't want to put the cart before the horse on salary negotiations, timing is very important.
Secondly, you state that she (wink, wink) does not currently have any other offer. If money is important to you, you might need to go to the second interview to keep this opportunity! However, if you are absolutely sure a better job will come along and that you can financially wait, etc - then this may not be necessary.
The next ingredient, you can state your intent that you would like to create a career plan within the company, from this position to end up at another position in the future. For example, during tester position interviews, we let them know that it is OK to use this position as a stepping stone and as long as you "put in your time". So if the position you are interviewing for has this type of flavor, it may be worth while to discuss this.
Furthermore, you mention turning down the second interview without burning any bridges...which is a red flag. Why would you really care about burning bridges if you have no desire at all to work there? To me it says you still have some inclination that for some reason you may still end up working there or near this group. With that said, I would go to the second interview for the sake of making a good impression, even though you know you may not accept the offer. This way you fulfilled all their interview requirements and did not shut them down too soon. When you get the offer you can say that you accepted another offer but if it doesn't work out would love the opportunity to be considered for the position again. As awkward as it sounds, it is the nature of business - I have seen people leave for better jobs and the boss himself says if it doesn't work out we would love to have you back! Crazy, I know.
In conclusion, you need to take these ingredients together and see what the whole picture is for you and remember: it's business, not personal.