Luis,
First off, while many will use a Steam Iron with a towel to open seams, unless you are doing this on woven goods, someone will damage the carpet and cause de-lamination.
A lot of the time, the problem with opening an existing seam, has to do with the poor way in which it was first put together. In our industry Installation is the number one complaint people have and the main point of contention has to do with seams. So this is a problem almost all of the time. Without going into a lot of why and what is not done or done improperly, what is important is to understand what is there and how to work with it.
This requires learning a good method of opening the seam, trimming it if needed and then knowing how to manipulate and work the seam. Actually this is fairly simple and even if the seam is 'butchered', one like yourself, can put it back together so that it looks good.
Even if someone opens the seam improperly and hacks it all up, you can often do wonders with it if you can learn how to control the situation as well as manipulate the seam where you want it to go. As my friend Tony often says, one has to 'Think outside of the box'.
Here is a little clip showing when the seam is 'peaked' it makes it very easy to cut the old tape and not the carpet, which if it is not peaked can be a little difficult for some.
Here is a clip showing how to take a 'butchered' seam and manipulate it to make it move where you want but also this allows you to control what you are doing. Notice only a small portion of this seam is being worked and pulled together, this was a very rough edge and had 3/4" gap to pull, but it can be done fairly easily
Your job is to learn some method you are comfortable with and either teach them how to properly open a seam, which will probably never happen, or you simply learn to manipulate a seam properly and control how you put it back together. This can be done even with some horrible seams. But save yourself some trouble and do not have them open seams with a steam iron.
Others made some suggestions that should work for you also.
I look at situations like this and consider it job security as well as a door opener.
SA