Mr Boney is just the game's "get out of gaol free card". Other DnD games have had the same idea of getting you back on your feet when you have cocked up; usually for money. Our saviour is clearly related to Jergal, since he spouts forth about erasing the dead character from the book of the dead when resurrecting.
It's cute that they use Jergal to link back to the BG1/2 story, but inconclusive as to whether this is the big J himself, or an important acolyte. I didn't really look closely at the Jergal statue in the crypt, but I did not get a sense that it exactly resembled Mr Boney, so they can probably make a decision on that later in development.
I would have to wonder ( at this point, anyway ) what Jergal would be doing loafing around in a minor crypt in the back of beyond, and why he would want to then loaf around in our camp. There would have to be a good rationale for such behaviour, so I would lean towards an acolyte ( who would naturally copy Jergal's characteristics ) being more reasonable, but who knows.
There are a couple of more reasons why I do believe it is Jergal (or at least his avatar), but that would go into datamined info and that is a whole can of worms that can very easily change. If you want to go looking at that, I would recommend it if only for the entertainment value.
I'll toss in a quick rebuttal to your statue idea. You can't take a statue as being entirely representative of who the subject of it was. Gods can take many forms, but it is the iconography that heavily implies that it is a statue dedicated to Jergal rather than a strict representation of him. This is further implied to be so as there are tapestries hanging on either side of the statue, depicting a skull with a scroll in its mouth, which is the symbol of Jergal. Artistic license and all that.
More to what I wish to say, you bring up a good point by asking why he is in that crypt. It is an important question but one I think I can give a possible answer to.
Our "talkative" friend, once we open his sarcophagus, says "So he has spoken, and so thou standest before me. Right as always." This seems to imply that he was directed to be there and await our arrival. We do not know how long he has been there, but he was likely waiting for some reason. On top of that, if you talk to him in camp, he says he responds with something along the line of "We meet again as predicted," and when you question why he is in your camp, he says "Be assured it is not by choice". From this, we can infer that this "friend" of ours was instructed to wait for us and offer services to us by his master. This master also seems to have the ability to predict the future. Drawing parallels to Jergal, Jergal doesn't care about the happenings of mortals and only wishes to carry on with his duties. However, Jergal does have a master, Kelemvor. Knowing Kelemvor, he has long opposed the Dead Three and can be reasoned that he tasked his servant to aid in ruining the schemes of Ban, Myrkul, and Bhaal. This could explain the general apathy he has for us and his motivation (or lack thereof). Now, it is hard to say whether or not Kelemvor or other gods truly have some form of precognition, but given how powerful they can be, it isn't hard to imagine.
Now, there are other theories that I have been contemplating and trying to reason out, so here is some additional thinking. For the sake of argument, let us presume that Mr. Boney is indeed Jergal. What if the entity or person that directed him to await us is not Kelemvor and who might it be? There is a theory going around that it might be Helm. Now, why would this be the case? Helm is one of the gods who has/does oppose the Dead Three, as they seek to disturb the peace and balance in Faerûn. One might speculate that Helm ordered Jergal to serve the party as penance for divulging much of his divine power to those three twisted adventurers long ago and causing the great unrest seen in the realms in recent history. Another theory stipulates that it might be Ao who ordered Jergal into this penance, given that it was Bane and Myrkul who stole the Tablets of Fate not long before the events of BG1. I am not sure if these would make sense but they do exist (if you look hard enough, *wink wink*).