In your own particular situation, we don’t know whether or not the wedding was presumptively valid because we don’t know whether the bride had the dispensations necessary to marry a non-Catholic in a non-Catholic ritual. Even presuming the wedding was presumptively invalid though, the Church does not explicitly forbid Catholics from attending invalid marriages and so Catholics must use their own prudential judgment in discerning attendance on a case-by-case basis. From what you’ve told me, it does not appear to me that you need to confess attending this wedding.
As for general rules:
*]Catholics may attend all presumptively-valid marriages of Catholics, non-Catholics, and non-Christians.
*]For Catholics marrying other Catholics or marrying a non-Catholic Christian or non-Christian, a wedding is presumptively valid if it is done in accordance with Catholic marital law. Catholics marrying non-Catholic Christians or non-Christians need a dispensation from cult to marry the non-Catholic party and a dispensation from form if they are marrying in a non-Catholic ritual.
*]For non-Catholics and non-Christians who are marrying other non-Catholics or non-Christians, a wedding can be considered presumptively valid if there are no known impediments to the marriage. The most common impediments that outsiders are likely to know about would be previous marriage, close blood relationship, or same-sex partners. If none of these impediments are known to exist, a prospective guest may presume that the wedding will be valid.
*]The Church does not explicitly forbid Catholics from attending presumptively-invalid marriages. Catholics must use their own prudential judgment in making the decision, keeping in mind the need to uphold the Catholic understanding of the sanctity of marriage. One rule of thumb that may be helpful in making such decisions might be to ask yourself if you believe the couple is doing the best that they can to act honorably and according to the truth that they have. So, for example, you might decide to attend the presumptively-invalid wedding of a couple who is expecting a child; but decline to attend the presumptively-invalid wedding of a couple who have engaged in adultery and destroyed previous marriages and families.
*]While there may be just reason to attend a particular wedding that will be presumptively-invalid, I cannot recommend participating as a member of the wedding party in such weddings. There is a difference between attending as a non-participating observer and actively involving yourself in the wedding as an honor attendant.
*]If you are not attending the wedding as a matter of principle, then I cannot recommend attending a reception or giving a gift to honor an occasion that you believe in conscience that you cannot celebrate. I do recommend though writing the couple a letter in which you express your love and that you will pray for them. (If prudence suggests it, it is fine to withhold from them what you will be praying to God that they obtain, such as the grace of repentance and conversion.)
*]In the case of same-sex partners, the Church has spoken so strongly against "same-sex marriage" that I cannot recommend attending or celebrating “same-sex weddings” under any circumstances.
Should I Attend?