I am not a canon lawyer, and Stack Exchange should not be relied upon for legal advice, but the canon lawyer Edward Peters does have something to say on this.
Canon 1432 of the Johanno-Pauline Code states “A defender of the bond is to be appointed in a diocese for cases concerning the nullity of sacred ordination or the nullity or dissolution of a marriage; the defender of the bond is bound by office to propose and explain everything which reasonably can be brought forth against nullity or dissolution.” There is no warrant here — or anywhere else in the 1983 Code — for Defenders of the Bond to make arguments for nullity.
Dignitas connubii 56 § 3 itself states “In every grade of trial, the defender is bound by the obligation to propose any kind of proofs, responses and exceptions that, without prejudice to the truth of the matter, contribute to the protection of the bond (cf. can. 1432)” and § 5 thereof drives home that “The defender can never act in favor of the nullity of marriage; if in a special case he has nothing that can be reasonably proposed or argued in favor of the bond, the defender can remit himself to the justice of the court.” Again, plainly, there is no canonical authorization for Defenders of the Bond to make arguments for nullity, and indeed, there is an express prohibition against such conduct.
Thus it would appear that there is no way for the Defender of the Bond licitly to do anything which might enhance the progress of the application for nullity.
It appears that some are interpreting an address by Pope Francis as an endorsement of the Defender of the Bond acting as a general counsel/adviser/Friend of the Tribunal, when he said
La sua presenza e il compimento fedele del suo compito non condiziona il giudice, bensì consente e favorisce l’imparzialità del suo giudizio, essendogli posti dinanzi gli argomenti a favore e contrari alla dichiarazione di nullità del matrimonio.
The official English translation of this says of the Defender of the Bond that
his presence and the faithful fulfilment of his task does not condition the judge, but rather allows and promotes the impartiality of his judgement by setting before him the arguments for and against annulment.
but it is doubtful that this is (a) a correct translation; (b) consonant with Canon Law. A reading which is consonant with Canon Law would be that the Defender sets forth the counter-argument to that of the applicant, thus ensuring that the judge has before him "the arguments for and against annulment." Pope Francis is not a canon lawyer, and his pronouncements and speeches have to be read and interpreted in the light of what has been formally promulgated by the Magisterium. Dignitas connubii 56 §5 specifically prohibits the Defender acting in favour of nullity.
I suppose that it is possible that a Defender of the Bond, having presented the argument against nullity, may be asked by the Tribunal to swap roles and stand as an adviser, as he is likely to be an expert in matters of nullity and relevant law and precedent. But in order to counter any perceived conflict of interest, if this is really necessary then good practice would surely be that this change of role should be made clear in the proceedings. Personally I feel that any dual role in proceedings is not good practice and the applicant should have their own expert [perhaps even appointed on their behalf by the Tribunal] to advise the judge on their application.
Addressing the question Is it his obligation or right to do so?, my answer is that not only does he not have an obligation to explain how best to interpret evidence for nullity, he actually has an obligation not to do that.